The present invention relates to field-effect transistors (FETs), more particularly, to a field-effect transistor having a channel region composed of an organic semiconductor layer, and still more particularly, to a thin-film transistor (TFT) having a channel region composed of an organic semiconductor layer.
In the related art, in thin-film transistors (TFTs), which are one type of field-effect transistor (FET), inorganic semiconductor materials, such as Si, GaAs, and InGaAs, are used as semiconductor layers constituting channel regions. In order to produce TFTs using such inorganic semiconductor materials, high-temperature processes at 400° C. or higher are required. Therefore, it is extremely difficult to form TFTs using inorganic semiconductor materials on a soft, crack-resistant, and light base (substrate), such as a plastic substrate.
On the other hand, TFTs in which channel regions are composed of organic semiconductor layers (hereinafter referred to as “organic TFTs”) can be produced at temperatures lower than the heat-resistant temperatures of plastics. Furthermore, such TFTs can be produced using coatable materials, and thus are expected to be used as inexpensive semiconductor elements suitable for application to large areas.
In organic TFTs to date, source/drain electrodes have been composed of a metal material, such as gold (Au), platinum (Pt), or palladium (Pd), so that good ohmic contacts are formed with organic semiconductor layers. Alternatively, the source/drain electrodes have been composed of poly(3,4-ethylenedioxythiophene)/polystyrene sulfonic acid [PEDOT/PSS], polyaniline doped with a dopant, or carbon nanotubes.
Gold, platinum, and palladium, which are metal materials capable of having good ohmic contact with p-type organic semiconductor layers, are expensive, and formation of source/drain electrodes and wiring by vapor deposition of such metal materials is not suitable for fabrication of low-cost organic TFTs. On the other hand, existing organic conductive material capable of forming a satisfactory ohmic contact with an organic semiconductor material have resistivities higher than those of the metal materials. Thus, such an organic conductive material is not satisfactory as a material for constituting source/drain electrodes and wiring.
An organic TFT having a structure in which source/drain electrodes composed of a metal material and an organic semiconductor layer constituting the channel region are not directly in contact with each other is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2003-229579 or 2004-103905.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 2003-229579, the source/drain electrodes each include a portion composed of a metal and a portion composed of a metal compound in contact with an organic conductive compound layer constituting the channel region. Examples of the metal compound include compounds containing metal atoms of groups 6 to 11 of the periodic table. Among these, metal compounds containing iridium, rhodium, ruthenium, platinum, gold, silver, samarium, osmium, palladium, nickel, cobalt, or europium, or a metal salt thereof are preferred. Alternatively, the metal compound may be selected from the metal complexes of these metals. (Refer to Japanese Unexamined Patent Application Publication No. 2003-229579, paragraph [0023].)
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 2004-103905, a buffer layer is disposed between a source/drain electrode and an organic semiconductor layer constituting the channel region, the buffer layer being composed of a metal oxide, such as ITO, IZO, tin oxide, or zinc oxide, a nitride, an oxide, a metal, an alloy, or an organic compound. (Refer to Japanese Unexamined Patent Application Publication No. 2004-103905, paragraph [0012].)
However, with respect to the metal compounds and the buffer layer disclosed in the Japanese Unexamined Patent Application Publications described above, the range of choice with respect to the conductivity type (n-type or p-type), resistance, etc. is not sufficiently wide. That is, design freedom is restricted with respect to the materials constituting the source/drain electrodes in organic TFTs.
It is desirable to provide a field-effect transistor including source/drain electrodes which can form good ohmic contacts with an organic semiconductor layer constituting the channel region, which are not required to contain an expensive material, such as gold, platinum, or palladium, and in which the range of choice with respect to the conductivity type, resistance, etc. is sufficiently unrestricted (i.e., design freedom is sufficiently large) with respect to the material used.